Described is an apparatus for guiding an electromagnetic microwave, having: antenna surrounding walls, which define an interior space so as to surround therein at least an end region of an antenna of a microwave source, in particular laterally annularly as well as frontally; waveguide boundary walls, at least two of which are arranged in parallel to each other, wherein the waveguide boundary walls form a, in particular cuboid-shaped, waveguide having a substantially rectangular cross-section, wherein a cross-sectional plane is defined by a first direction that extends along a longitudinal direction of the antenna and a second direction that extends perpendicularly to the first direction, wherein it holds: 25>a/b>3, wherein a: is a width of the waveguide along the second direction, b: is a height of the waveguide along the first direction, wherein the apparatus is designed to let proceed a microwave from the interior space of the antenna surrounding walls into the waveguide.